Strands vs. crosslinks: topology-dependent degradation and regelation of polyacrylate networks synthesised by RAFT polymerisation

Degradable poly(n-butyl acrylate) networks were synthesised by reversible addition–fragmentation chain transfer (RAFT) polymerisation using a cleavable disulfide diacrylate crosslinker or a cleavable comonomer, dibenzo[c,e]oxepine-5(7H)-thione (DOT). Through the analysis of gelation kinetics, equilibrium swelling ratio and storage modulus, it was found that incorporation of the degradable units in both cases did not significantly impact the mechanical properties of the prepared gels compared to non-degradable controls. While both types of networks were found to readily degrade, either by thiol–disulfide exchange or aminolysis, they produced degradation fragments of different topology, namely monodisperse linear chains in the case of degradable crosslinkers and branched polydisperse fragments in the case of degradable strands. A simple oxidation of thiols to disulfide bonds in air at 30 °C was successfully used to repolymerise both types of degraded fragments back to solid networks through multiple degradation/regelation cycles. Interestingly, the branched, disperse fragments from degradation of the DOT-containing networks repolymerised more readily and produced networks with properties closer to the original material. This is in contrast to polyacrylate gels made by conventional free radical polymerisation (FRP) which do not degrade through cleavable crosslinks, only through cleavable strands. However, these networks cannot successfully reform after degradation. Furthermore, the apparent dependence of the regelation efficiency on topology of the degraded fragments can open a pathway to better understand reprocessing and recycling of crosslinked polymer networks.